The brake system is all drums, with a 71 or 72 dual master cylinder.
The right rear bled OK.
The left rear just keeps putting out a steady stream of bubbles.
Where are they coming from?

I swapped in a new left rear wheel cylinder, and it does the same thing.
How do I find the leak?
Is there a way one person can do it?

Been working on it a couple weeks...

I got air in the system when bleeding the left front.

I was using a little one-man bleeding kit.
One time, I got on the ground and closed the bleeder,
emptied the jar,
forgot to top-off the master cylinder,
got back in the seat,
and the pedal went to the floor.

That's when I started bleeding the rear brakes and ran into this problem.

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63 Signet / 10 inch front drums / weber 2bbl / HEI / 2.76 rear end

you need to bench bleed the m/c again, have someone pump the brakes while you crack the lines at the m/c--- get all the air out of there 1st-- then proceed with the rear most point.
If that doesn't work check your fittings closely== maybe a hairline crack in the lines , Lawrence